Real-time CORBA is a middleware standard that has demonstrated successes in developing distributed, realtime, and embedded (DRE) systems. Customizing real-time CORBA for an application can considerably reduce the size of the middleware and improve its performance. However, customizing middleware is an error-prone task and requires deep knowledge of the CORBA standard as well as the middleware design. This paper presents ZEN-kit, a graphical tool for customizing RTZen (an RTSJ-based implementation of real-time CORBA). This customization is achieved through modularizing the middleware so that features may be inserted or removed based on the DRE application requirements. This paper presents three main contributions: 1) it describes how real-time CORBA features can be modularized and configured in RTZen using components and aspects, 2) it provides a configuration strategy to customize real-time middleware to achieve low-footprint ORBs, and 3) it presents ZEN-kit, a graphical tool for composing customized real-time middleware.